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LTM8042/LTM8042-1

80421fa

OPERATION

The LTM8042/LTM8042-1 is a complete, full featured,

current mode regulator specifically designed to drive light

emitting diodes (LEDs) or other loads where a constant cur-

rent up to 1A (350mA for the LTM8042-1) is required.

The LTM8042/LTM8042-1 can operate in any of three LED

drive topologies: boost, buck mode and buck-boost mode.

The device features both analog and PWM dimming, a PWM

P-channel MOSFET driver, and a suite of control functions:

RUN control, soft-start, user programmable switching

frequency, and external frequency synchronization.

Operation can be best understood by referring to the

Block Diagram. The power stage is a boost converter that

regulates the output current by reading the voltage across

a power sense resistor that is in series with the output.

As with any boost topology, there is an uninterrupted cur-

rent path between the input and output terminals. Current

between these two terminals is not limited, so the device

is intolerant to a short-circuit or overload from any of the

output terminals (LED+, BSTOUT/BKIN) to GND.

There are two ways to dim a LED with the LTM8042/

LTM8042-1. One way is to adjust the current on the LED

array by setting the analog voltage on the CTL pin. The

CTL pin is internally pulled up to a precision 2V reference

through a 1% 20k resistor. Leaving the CTL pin floating

sets the LED pin current to 1A. Reducing the voltage below

1.1V on the CTL pin proportionally reduces the current

flowing out of LED+. This can be accomplished by connect-

ing a resistor from the CTL pin to GND, forming a divider

network with the internal 20k resistor, or by driving the

CTL pin directly to a voltage source, such as a DAC.

The other way the LTM8042/LTM8042-1 can dim a LED

array is by pulse width modulation using the PWM pin

and an optional external P-channel MOSFET. The external

P-channel MOSFET can be conveniently operated by the

integrated gate driver at pin TG. The gate drive function

can be enabled or disabled by the TGEN pin.

If the PWM pin is pulled high, the part operates nor-

mally. If the PWM pin is unconnected or pulled low, the

LTM8042/LTM8042-1 stops switching and the internal

control circuitry is held in its present state. This way, the

LTM8042/LTM8042-1 “remembers” the current sourced

from the LED+ output until PWM is pulled high again.

This leads to a highly linear relationship between pulse

width and output light, allowing for a large and accurate

dimming range.

The RUN pin is used to deactivate the LTM8042/LTM8042-1.

When the RUN pin is pulled to a logic low state, the device

is shut down and draws typically less than 1μA of current.

The SS pin is used to limit inrush current during start-up.

The LTM8042/LTM8042-1 integrates a current source with

this function, so only a capacitor is necessary to establish

the soft-start characteristics of the output current.

The switching frequency is set by applying a single resistor

from the RT pin to GND, allowing operation anywhere from

250kHz to 2MHz, and the SYNC pin allows synchroniza-

tion to an external source between 300kHz and 2.5MHz.

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